Magnesium oxide is an inorganic compound excellent in thermal conductivity, thermal resistance, and the like, and is used in various resins as a thermally conductive filler for enhancing thermal conductivity of a resin composition. When magnesium oxide is used as a thermally conductive filler, its moisture resistance (also referred to as slaking resistance) is a problem. Conventionally, the surface of magnesium oxide is coated with an oxide such as silicon oxide or a resin such as epoxy or silicone, thereby preventing water from contacting with the magnesium oxide.
For example, Patent Literature 1 describes that a magnesium oxide filler of an electron emission element is coated with silicon oxide (SiO2), thereby enhancing the thermal conductivity of the electron emission element. Also, Patent Literature 2 describes that a coating layer of silicon or the like is formed on the surface of magnesium oxide powder, to form a resin composition excellent in moisture resistance and thermal conductivity.